A. Field of the Invention
This invention relates to the field of metallized film wound capacitors and particularly to high current capacitors used in high frequency applications.
B. Prior Art
Every capacitor has a certain amount of inherent inductance. This inductance, combined with the intended capacitance, forms a resonant circuit. Higher unintended inductance causes the capacitor to resonate at a lower frequency, thus limiting the usefulness of the capacitor for high frequency applications. Thus, many capacitors which are adequate for routine applications may not be adequate in the portion of the radio frequency range above 50 kilohertz for example. In high current applications, such as switching power supplies, where capacitors have a large physical size and a consequently lower equivalent series resistance, this problem is critical. In order to limit this undesired parasitic inductance inevitably created when making a capacitor, various physical structures have been used to cause the inductive reactance of various sections of the capacitor to cancel.
A successful method previously used to limit the inherent inductance of capacitors took advantage of the physical principle that two parallel conductors carrying essentially the same current in opposite directions create approximately equal electromagnetic fields whose directions are in opposition. The closer in physical distance that these two parallel conductors were placed the more nearly completely the electromagnetic fields cancelled each other. The theoretical ideal would be achieved if the two conductors occupied the same physical space. A close approximation to this ideal would be to place one conductor entirely inside the other. This has been effectively achieved by completely enclosing the wound capacitor in a metal can. One electrode of the capacitor was internally electrically connected to one end of the can and the other electrode was electrically connected to a lead which exited the can at the opposite end. The wound capacitor itself may be conceptualized as one conductor and the metal cans surrounding it may be conceptualized as a second conductor thereby creating two parallel conductors, one within the other, constantly carrying current in opposite directions. While this arrangement effectively minimized inherent inductance, thereby increasing resonant frequency, it was undesireably expensive and inefficient in the amount of capacitance per unit volume.
The following patents were located in a search directed to this invention but upon review were not found to be relevant:
______________________________________ U.S. Pat. No. Inventor(s) ______________________________________ 3,156,854 H. Beyer 3,153,180 A. J. Bellmore 3,163,917 D. L. Bilsing et al. 3,149,398 Sprague et al. 3,292,053 A. DiGiacomo 2,985,803 J. B. Brennan ______________________________________
It is therefore an object of this invention to minimize inherent inductance by effectively placing one conductor within a second conductor thereby cancelling inductive reactance and maximizing the self resonant frequency.
A further object is to optimize the cancellation of inductive reactance relatively inexpensively.
A final object is to provide a capacitor with a large value of capacitance with high current carrying capability which may be operated at a high frequency.